The present invention relates to a fixing device to fix an image formed on a sheet and an image forming apparatus.
In recent years, there have been increasing demands of outputting high quality image and long lifespan of parts related to the electrophotographic process for image forming apparatuses of electrophotographic method such as printers and digital multifunction peripheries from the market.
In response to such demands from the market, there is suggested a technology which distributes abrasion of the fixing roller caused by the side edge section of the sheet repeatedly passing through the fixing apparatus so that the abrasion does not concentrate on one portion of the fixing roller surface (Patent documents 1 to 3: Unexamined Japanese patent application publication No. H9-265219, 2003-263090 and 2007-148336). As a result, deterioration of an image (smear and uneven glaze) caused by abrasion of the roller surface and early replacing of the fixing roller due to the deterioration of the image can be obviated and longivity is realized.
In an image forming apparatus of Patent Document 1 (Unexamined Japanese patent application publication No. H9-265219), on an upstream side of a fixing device, there is disposed a transfer conveyance mechanism to convey a transfer sheet after skewing the sheet by a predetermined angle with respect to the fixing roller. Whereby, when one piece of the transfer sheet passes through the fixing roller, the transfer sheet is conveyed while a contact portion between the side edge section of the transfer sheet and the fixing roller is moving in an axis direction of the fixing roller. As a result, the contact portion of the fixing roller with the side edge section of the transfer sheet changes continuously, thus local abrasion is obviated.
An image forming apparatus described in Patent Document 2 (Unexamined Japanese patent application publication No. 2003-263090), on an upstream side of the image forming section, there is disposed a position changing device which enables to change a position of the transfer sheet in a width direction so that a starting position of writing in the image forming section is changed in accordance with a position of the transfer sheet to be conveyed to the transfer section. As a result, the side edge of the sheet entering into a nip section of the fixing roller shifts in the width direction relatively so as to obviate local abrasion of the transfer fixing body caused by the side edge of the sheet.
An image forming apparatus described in Patent Document 3 (Unexamined Japanese patent application publication No. 2007-148336) displaces a positional relation between the sheet conveyed from the sheet feeding apparatus and the transfer fixing body of the transfer fixing apparatus in the width direction relatively so that the local abrasion of the transfer fixing body caused by the side edge of the sheet is obviated.    Patent Document 1: Unexamined Japanese patent application publication No. H9-265219    Patent Document 2: Unexamined Japanese patent application publication No. 2003-263090    Patent Document 3: Unexamined Japanese patent application publication No. 2007-148336
Meanwhile, the inventions of the above patent documents have the following problems.
In the Patent document 1, the sheet as a whole skews between the fixing apparatus and the transfer section, and the sheet having been skewed is conveyed in the sheet conveyance direction while maintaining the posture of the sheet as it is. Thus, a complicated and large transfer sheet conveyance mechanism will be necessary. Therefore, there are problems that the cost increases and the apparatus grows in size.
In the Patent Document 2, a position changing device to displace the whole sheet in the width direction is needed between the transfer section and the sheet feeding apparatus, thus the cost increases and the apparatus glows in size in the same manner as the Patent document 1. Also, since a temperature distribution of the fixing roller with respect to the sheet entering into the fixing device changes, a balance of amount of heat supplied to each section of the sheet is not steady, thus fixing failure such as uneven fixing (graze and degree of fixing) and crinkle of the sheet have occurred.
In the Patent Document 3, since the transfer fixing body displaces in the width direction relatively with respect to the sheet, a problem of uneven fixing (graze and degree of fixing) of the toner image on the sheet exists because of the same reason as the Patent Document 2. Also, since it has a structure that only the transfer fixing body is displaced in the width direction with respect to the sheet entering into the transfer fixing device, a mechanism to separate the pressure roller from the transfer fixing body when the transfer fixing body is moved or a mechanism to move the transfer fixing body in the axis direction by a strong force against a pressure of the pressure contact roller is needed, thus there is a problem in a view point of a practical application.
The present invention has one aspect to resolve the above problems and an object of the present invention is to provide an image forming apparatus capable of attaining longevity of the fixing roller without having problems of uneven fixing and the crinkle of the sheet by stabilizing balance of the amount of heat supplied to each portion of the sheet and obviating local abrasion to occur on the fixing roller.